1. Field of the Invention
This invention relates to an acceleration sensor for detecting an acceleration of a system, wherein the acceleration sensor is also accelerated, and to a method for detecting an acceleration.
2. Description of the related art
For detecting an acceleration of a moving system, sensors (acceleration sensors) can be used, which are integrated in the moving system. Thereby, the acceleration sensors are also accelerated, so that normally no external points of reference can be used for detecting the acceleration. Thus, a central part of every acceleration sensor is an element changing one of its physical characteristics due to the external acceleration. If the acceleration sensor has, for example, a spring-mass system, a position of the mass suspended above the spring can be used for determining the acceleration in relation to a fixed point of the sensor. Additionally, there is also the possibility to utilize a change of electrical, magnetical or also optical characteristics of an element contained in the acceleration sensor when an external acceleration is applied, for determining the same.
Known technological solutions, which are based on the usage of a micromechanically produced spring-mass system, are, on the one hand, based on a detection of a deflection of a mass, which remains in a resting position by a mechanical restoring force of the spring when no external acceleration is applied, as described in the paper of S. Bütefisch et al.: “Three Axis Monolithic Silicon Low-g Accelerometer”, J. Micromech. Systems, vol. 9, no. 4 (2000), pp. 551–556. When an acceleration is applied, the mass is deflected into a position which is given by an equilibrium of forces between spring restoring force and accelerating force. The acceleration is determined based on the deflection.
Apart from that, solutions exist where the accelerating force is compensated, for example, by an electrostatic force, such that the mass is substantially in its resting position, as it is described in the paper of N. Yazdi et al.: “An All-Silicon Single-Wafer Micro-g Accelerometer with a combined Surface and Bulk Micromachining Process”, J. Micromech. Systems, vol. 9, no. 4 (2000), pp. 544–550, as well as in U.S. Pat. No. 5,540,095 by Sherman et al.: “Monolithic Accelerometer”. In that case, the required compensation force represents a measure for the applied external acceleration.
In both cases, normally, the deflection caused by an external acceleration of a resiliently mounted seismic mass is determined such that a change of capacitance in connection with the deflection is determined via a differential capacitor means, or maintained at zero by electrostatical forces, respectively.
However, it is disadvantageous of the above-described approaches that they are susceptible to temperature variations as well as a possible drifting of the amplifier elements. Thus, a read-out circuit has to be switched such that such parasitic effects are compensated internally. This leads to an increased complexity of the read-out circuit as well as to possible measurement inaccuracies, which can have devastating consequences, particularly when using the acceleration sensors in the field of automobile safety technology. If acceleration sensors structured that way are, for example, used for vehicle stabilization, inclination measuring or also as airbag and impact sensors, respectively, possible measurement inaccuracies as well as the plurality of error sources always linked to the complexity of the read-out circuit, will cause, for example, vehicle malfunction in dangerous situations or also no release of the airbag.